fix(staleness): use subtraction to avoid overflow for large timeout values

[thedavidmeister]

Summary

block.timestamp > timestamp + timeout overflows when timeout is near type(uint256).max, causing a checked-arithmetic Panic instead of a clean pass. A very-large timeout semantically means "never stale," but the overflow makes the check revert instead.

Fixed in both stale-price checks:

• src/lib/lts/LibFtsoV2LTS.sol: ftsoV2LTSGetFeed (timestamp is uint64, promoted to uint256)
• src/lib/price/LibFtsoCurrentPriceUsd.sol: ftsoCurrentPriceUsd (both uint256)

Replacement: block.timestamp > timestamp && block.timestamp - timestamp > timeout — subtraction avoids the addition overflow entirely. The first guard (block.timestamp > timestamp) is always true for real feed data and makes the unsigned subtraction safe.

Test plan

• Existing staleness fuzz tests still pass (they bound timeout to type(int224).max which avoids the old overflow, so behavior is unchanged for those inputs)
• CI rainix-sol-test passes

Closes #102

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 51cce43e-67b2-4d0d-8956-4046afe7a070

📥 Commits

Reviewing files that changed from the base of the PR and between cb20d03 and aadcf61.

⛔ Files ignored due to path filters (1)

• src/generated/FlareFtsoWords.pointers.sol is excluded by !**/generated/**

📒 Files selected for processing (2)

• src/lib/lts/LibFtsoV2LTS.sol
• src/lib/price/LibFtsoCurrentPriceUsd.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-102-staleness-overflow

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